Future Heat Stress Projections in Northwestern Türkiye: Urbanization and Population Impacts in Istanbul

The frequency and intensity of heat stress are expected to escalate markedly in the near future under various global climate change scenarios, with densely populated cities becoming hotspots because of the urban heat island effect. Therefore, heat stress analysis for highly populated cities is crucial since changes in this stress exacerbate vulnerability, increase health-related risks and impose constraints on outdoor activity. This study investigates changes in heat stress during 21^st century in terms of frequency, intensity and durations while quantifying population exposure to heat stress covering Northwestern Türkiye, with particular attention to Istanbul, the most populous city in Türkiye with nearly sixteen million population.

In this study, we use climate simulations from convection-permitting model COSMO-CLM under SSP3-7.0 emission scenario to investigate future changes in heat stress. The analysis focuses on calculating Wet Bulb Temperature (WBT) values and assessing consecutive hours when Wet Bulb Temperature (WBT) is above specific thresholds, which are critical indicators of heat stress severity. In addition, we conduct comprehensive heat stress evaluation by computing Environmental Stress Index (ESI) values, an effective alternative to WBGT, to assess outdoor activity limitations. These analyses are performed for the reference period of 1985-2015 and extended to future periods of 2030-2039, 2050-2059, 2070-2079 and 2090-2099, providing a detailed temporal perspective on the progression of heat stress and its implications under changing climatic conditions.

WBT uses air temperature and relative humidity as its primary parameters while ESI incorporates radiation alongside air temperature and relative humidity. Thus, this study also comprehensively analyzes the role of radiation in amplifying heat stress. Our results reveal a remarkable seasonal shift in heat stress pattern within the study area with Istanbul standing out as a hotspot where heat stress indices are notably higher than those of other cities in the covered region, highlighting the effect of urbanization in heat stress dynamics.

Notably, ESI values in the southern parts of Istanbul, where urbanization is more concentrated, exceed critical thresholds that makes any physical activity to be hazardous especially by the end of this century. Moreover, projections demonstrate that in the late 21^st century, majority of Istanbul’s population will be exposed to heat stress levels exceeding the risky thresholds. Furthermore, this study explores the extent of population exposure to heat stress, the duration of consecutive hours exceeding critical thresholds, and the percentage of areas where indices exceed their limits.

Key words: Climate modelling, heat stress, heat extremes, population exposure, COSMO-CLM